The natural mineral zeolite (e.g. porous aluminosilicate) has the property of strongly adsorbing water vapor, incorporating it in its internal crystal lattice, with a significant release of heat. Water can be deabsorbed from zeolites by heating. The absorption process can proceed in a partially evacuated environment where the absorption of the water by the zeolite can be so forceful that the internal pressure of the closed system drops dramatically and causes liquid water in the system to evaporate and cool, even to the point of freezing. This effect can be used to generate a cooling system where water is evaporated from one chamber of the system, cooling that chamber, and absorbing into the zeolite until the zeolite has reached its saturation point. This characteristic of zeolites has been explored for the development of air conditioning systems, particularly for systems where portability is desired. Portable systems can be used for mobile individuals in occupations where they must wear protective clothing in hot environments or for automobiles, where a significant amount of waste heat is available for the removal of water to recover the water absorbing zeolite.
The water adsorption capacity decreases as the temperature rises in a zeolite and limits the performance of a cooling system designed using a zeolite absorber. For systems of this type, a significant effort has been directed at the surface area of the zeolite bed in contact with the outside surface with the goal of increasing the rate of heat loss from the zeolite absorber. Many studies have focused on the design of the bed such that it has more surface area on the outside so that cooling by outside air will be more efficient. Unfortunately, the improvement of the absorption capacity has not been significant. A limitation for such a system is the thermal conductivity of the zeolite bed itself. A conventional packed bed of zeolite beads or powder is very thermally insulating with a room temperature thermal conductivity of about 0.2 to no more than about 10 W/m·K. The heat generation on absorption of water far exceeds heat loss to the environment for the best external architectures. Zeolite beds can reach temperatures of about 120° C. or more while absorbing water at practical rates of absorption for such cooling systems.
Zeolites are often employed as absorbents in air purification systems. Zeolites have been employed with other absorbents to enhance or modify the systems. A commonly employed combination is that of carbon and zeolites, which are considered complimentary absorbents. Usually they are employed in discontinuous layers, but have been used as a mixture of solids. The “absorbent carbon” used in such systems are charcoals and activated carbons formed by the pyrolysis of coal, wood, bark, and other cellulose based materials where volatile materials are remove. These “absorbent carbons” are also thermal insulators with thermal conductivities in the range of 0.5 W/m·K. Therefore, these mixed absorbents cannot significantly improve the water capacity limitations of the zeolites due to the high temperatures achieved by the zeolites during absorption.
There is a need to address absorption limitations due to the heating of the zeolite upon absorption of water. A route to improve the cooling of the zeolite during water absorption is also needed such that cooling and other systems based on the absorption of water in zeolites might be significantly enhanced.